1. Field of the Invention
The invention relates to interrupt systems in computers, and more particularly, to filters for removing noise from interrupt request signals and extending signals to assure proper interrupt operation.
2. Description of the Related Art
Modern computer systems often employ a wide array of peripheral components and I/O devices. The added components provide the computer system with versatility and capability that tremendously enhance the computers performance and effectiveness. Current devices include devices like printers, communication ports, scanners, plotters, and hard drives.
Each of these I/O devices must communicate with the computer system through an I/O interface. Each peripheral component preferably generates interrupt requests to the microprocessor to demand the microprocessor's attention for particular tasks. If the devices could not interrupt the microprocessor, the computer system would have to poll the status of each I/O device at specific intervals to determine whether each device had developed a need for the microprocessor's attention. In most cases, the status check is unnecessary, resulting in wasted time. Using the interrupt system, however, the microprocessor spends no time checking the I/O device's status until an interrupt request is asserted, demanding the microprocessor's attention.
Interrupt requests are generated by the I/O devices and are provided to the computer system through the I/O interface. Each time an interrupt is asserted and recognized, the microprocessor pauses its current operations and executes an interrupt routine to service the I/O device's needs. When the interrupt routine is complete, the microprocessor returns to its original task until the next interrupt is recognized.
Although I/O devices may generate the interrupt signals to service their needs, interrupt request signals may also be generated inadvertently by noise in the environment. Normally low, positive edge triggered signals in TTL environments are particularly susceptible to noise interference. Each time the interrupt signal is asserted and recognized by the microprocessor, the microprocessor must pause its current operations and execute the interrupt routine. If the interrupt signal is generated by noise, the I/O device does not require service, and the execution of the interrupt routine serves no purpose. The microprocessor must then finish the interrupt routine and return to its original task. Each time an interrupt signal is inadvertently asserted and recognized by the system, the microprocessor wastes valuable time and suffers a reduction in efficiency.
Furthermore, some interrupt signals are normally maintained at a logic level high. To assert the interrupt, a negative pulse is asserted at the interrupt input. For many interrupt signals, the interrupt is activated by the rising edge of the negative pulse. Others are level-sensitive logic low signals. If the negative pulse is too short, however, the low logic level of the interrupt signal may not be detected by the computer system, and thus the rising edge, or the low level, of the signal is not detected. Consequently, the interrupt request is not serviced and the I/O device does not receive the necessary assistance from the microprocessor.